Wireless communication systems, such as the 3rd Generation (3G) of mobile telephone standards and technology, are well known. An example of such 3G standards and technology is the Universal Mobile Telecommunications System (UMTS™), developed by the 3rd Generation Partnership Project (3GPP™) (www.3gpp.org). The 3rd generation of wireless communications has generally been developed to support macro-cell mobile phone communications. Such macro cells utilise high power base stations (NodeBs in 3GPP™ parlance) in order to communicate with wireless communication units within a relatively large geographical coverage area. Typically, wireless communication units, or User Equipment (UEs) as they are often referred to in 3G parlance, communicate with a Core Network (CN) of the 3 G communication system via a Radio Network Subsystem (RNS). A wireless cellular communication system typically comprises a plurality of radio network subsystems, each radio network subsystem comprising one or more (coverage) cells to which UEs may ‘attach’, and thereby connect to the network. Each macro-cellular RNS further comprises a controller, in a form of a Radio Network Controller (RNC), operably coupled to the one or more Node Bs, via a so-called Iub interface.
Lower power (and therefore smaller coverage area) femto cells (or pico-cells) are a recent development within the field of wireless cellular communication systems. Femto cells or pico-cells (with the term femto cells being used hereafter to encompass pico-cells or similar) are effectively communication coverage areas supported by low power base stations (otherwise referred to as Access Points (APs) of Home Node B's (HNBs)). These femto cells are intended to be able to be piggy-backed onto the more widely used macro-cellular network and support communications to UEs in a restricted, for example ‘in-building’, environment.
Typical applications for such femto APs/HNBs include, by way of example, residential and commercial (e.g. office) locations, communication ‘hotspots’, etc., whereby APs/HNBs can be connected to a core network via, for example, the Internet using a broadband connection or the like. In this manner, femto cells can be provided in a simple, scalable deployment in specific in-building locations where, for example, UEs may come into close proximity to a femto AP/HNB. Thus, femto APs/HNBs are intended to enhance the coverage of a UMTS™ Radio Access Network (RAN) within residential and/or private commercial environments, and it is planned that the number of femto HNBs in a macro cell may number thousands.
If required, the location of an AP is typically measured using a GPS module that is part of the AP. In a femto cell network it is known that there may be a very large number of femto cells compared to the number of macro cells, with femto cells often residing within or overlapping with macro cells in the same geographic area. Thus, the coverage area of a single macro cell will inevitably overlap (and encompass) a coverage area of a large number of femto cells. Both macro cells and femto cells need to be configured with their individual cell parameters and optimised to balance the operators overall network performance. Individual macro cells which cover a larger population area are carefully configured, tuned and monitored using ‘drive tests’ and network performance measurements. Drive-testing and analysis of the results of the individual macro-cells is an expensive and time-consuming activity. Furthermore the drive tests are typically performed from inside a vehicle travelling on roads. The RF environment of a macro cell is usually defined by the measurements collected in drive test campaigns. Information about the RF environment of a macro cell is also reported in the measurement messages sent by UEs. However, femto cell APs tend not to be optimised on an individual basis due to their sheer number. APs must perform their own parameter optimisations (and are so called ‘Self optimising’).
One such configuration is the neighbour cell list. Traditionally in femto cells these are statically configured via O&M or use techniques such as Radio Environment Monitoring (REM) or network listen (NWL) to detect macro cells automatically and/or to determine the AP's own operational parameters. Additionally APs are capable of capturing other logs and performance measurements (such as call drop rates, handover attempts, voice quality related parameters, data throughput parameters as just some examples). Users typically spend their time indoors inside buildings and as such measuring the coverage of the macro cells inside a building requires ‘walk-testing’ which is even more costly to cover an area. Also, due to the placement of the AP indoors there is a chance that the AP cannot detect all of the available NCells outside whereas the UE (which is mobile) may find additional NCells. In WCDMA 3GPP the UE is only able to report the Frequency and Scrambling code of cells it detects over the air. In order to use this cell for handover, the AP must know its CellID as well so that it can inform the core network of this.
To obtain the CellID the small cell subsystem generally requires costly integration with other parts of the network such as the macro cell OA&M system. Typically the components of the Cell Planning tools, provisioning systems are developed by different vendors and as such agreeing APIs and overcoming inter-operational issues is costly. In addition keeping cell planning databases accurate and up to date provides the operator with an operational cost (OPEX) to maintain this information.
In an alternative aspect, in a macro cell environment, a Network Operator usually tries to include as many overlapping cells as possible in the neighbour list of a NodeB sector (although there can be exceptions where the maximum possible size of the neighbour list might be reduced). This is mainly driven by the micro-diversity advantage of soft handover in a WCDMA system. The larger the active set of a UE connection the less likely a connection will be lost or severely affected when the connection with one member of the active set is lost or degraded.
Macro cell neighbours are typically planned using static planning tools which use empirical propagation models to simulate the coverage characteristics of the cells and rank neighbour cells based on the probability of overlapping with a source cell. Although ranking is typically generated by the planning tools it is usually not a critical element in the neighbour planning process unless the number of overlapping cells exceeds the maximum neighbour list capacity.
Moreover, neighbour lists can go through rounds of optimisation which include collecting drive test measurements to identify missing neighbours and, to lesser extent, unneeded neighbours. Some vendors offer the capability of collecting and post-processing UE measurements in real time or through an offline optimisation process to identify missing or unneeded neighbours.
Missing neighbours of macro cells are typically added to the macro cell neighbour cell lists as normal neighbours for there is no need to differentiate between them and existing neighbours except in situations where the addition of missing neighbours may result in exceeding the neighbour list maximum capacity in which case neighbours are typically prioritised based on overlapping statistics (predicted through simulations and/or measured through drive tests or UE reports).
Information used in the macro cell neighbour planning or optimisation (simulated or measured) typically only applies to the corresponding cell due to the lack of correlation between macro cells.
Femto cells on the other hand do not use soft handover but rather hard handover. This is true for at least the vast majority of currently deployed femto cells (residential or enterprise). Handout from femto to macro is supported, whereas hand-in from macro to femto often isn't supported. It is therefore critical for a femto cell to have its neighbour cell list reliably configured to reduce the risk of a call drop during or after a handover. If some form of neighbour list optimisation is introduced to the femto cell to find missing neighbours it can be risky to add those neighbours directly and permanently to the neighbour list as the addition of an unreliable neighbour can result in increasing the call drop rate on the femto cell or the target macro cell. Assessing the reliability of potential neighbours might therefore include configuring the UEs to report measurements on those neighbours without initiating handovers to them. This will add an additional burden on the UE especially if handover speedup is critical for the survival of a connection. Some mechanism to prioritise those potential neighbours for the purpose of including them in the assessment is required in order to avoid wasting the UE's resources and time in assessing potential neighbours that are more likely to be unreliable.
Moreover, due to the nature of the femto cell environment where one or more indoor barriers can exist a sudden change in the RF environment is very likely and the femto cell may be required to react quickly to that change by initiating a handover to another cell. One factor that can affect the speed of the UE's reporting of neighbour measurements is the number of neighbours included in the measurement control messages by the cell. It is therefore important that the femto cell optimises the size of its neighbour list to avoid lengthy measurement processes by the UEs.
Due to the correlation that might exist in the RF environments of femto cells within close vicinity of one another, information measured or reported by a femto cell can be relevant in the neighbour optimisation process of the other surrounding femto cells.